Concrete Materials and Admixtures

The Risks of Low-Cement Concrete

{QUESTION} We have had a request to make concrete with a ternary blend—cement, fly ash, and slag—with only 25% cement. What should we be concerned about? What precautions should we take?

{ANSWER} There has been a lot of use of high-volume replacements with pozzolans such as fly ash and slag in concrete with the goal of improving performance and sustainability in the hardened state. This has resulted, in some cases, in issues with construction, including setting, strength gain, slump, color, and air-content retention. As a producer, one must know how these items will affect the concrete before the date of the first pour.

Don’t just take one of your existing mixtures and add the pozzolans. This would be a disaster. High-volume pozzolan concrete is a very different material in the plastic state due to the nature of the binder. The first thing to do is to design the mixture using a three-point water-cement ratio curve. Remember that having fly ash in the mixture will act as a water-reducer, so a lower-than-normal water-cement ratio could be used. Take care not to have a water-cementitious ratio lower than 0.30 though. Below that, you run a good chance of getting autogenous drying and a lot of shrinkage. On that basis you can set the proportions.

I hope you are still reading after seeing that low water-cementitious material ratio. Keep that distinct from the water content. For that, you need at least 220 pounds of water, and maybe higher for pumping. So even at 0.30, there needs to be at least 660 pounds of total cementitious materials.

There are two major classes of fly ash, each differing in nature. Class C fly ash is a hydraulic cement and will contribute to setting and early strength gain. Class F fly ash is strictly a pozzolan and will be somewhat inert from the point of view of setting. Slag is also strictly a pozzolan and comes in three grades: 80, 100, and 120. Generally, the higher number indicates a higher strength at later ages, all things being equal. Slag is sensitive to the alkali content of the cement (and fly ash) and is sensitive to temperature. So if your cement is a low alkali (Na2O) it will gain strength slower with slag than with a higher alkali content cement.

Setting times

A consequence of the pozzolanic nature of the cementitious materials is that they will not set quickly. If exposed to cold temperatures they will be more affected than the usual concrete you use. Similarly, the concrete will not set as quickly at high temperatures; as a result, plastic shrinkage and evaporation may be a major concern. Accelerators (non-chloride if the concrete contains steel reinforcement) may be required, even at temperatures where they’d normally not be used.

The same is true for concrete in test cylinders. Treat the cylinders more gingerly than normal, as they may be reduced in strength gain. Similarly, the setting of concrete will be slower, possibly resulting in higher form pressures and reduced rate of rise in the forms.

Admixtures are often surface active materials. The surfaces of the slag and fly ash may be different from the cement you usually use, so try your admixtures to make sure there are no incompatibilities. Some water reducers will cause retardation in setting in high-level replacement mixtures. Some fly ash can contain carbon from the coal that will absorb the admixtures. This can sometimes be identified by a high loss on ignition. Air entraining agents are most often affected, and it can be hard to entrain air in high-volume pozzolan concretes.